This invention relates generally to a steering unit for barges and other watercraft, more particularly to a means for priming the blades of an axial flow pump utilized for steering and maneuvering the watercraft.
In my prior U.S. Pat. No. 3,590,766, which adopts the axial flow pump principle set forth in my U.S. Pat. No. 2,847,941, a steering unit is disclosed for barges and the like as including a transversely extending casing having an axial flow pump therein for moving water through the casing in selectively opposite directions so as to steer and maneuver the barge in corresponding opposite directions. The casing has an intermediate portion connected with downwardly and outwardly extending end portions communicating with spaced openings in the bottom hull. A shroud structure surrounds the blades of the pump and has two axially juxtaposed, annular inwardly directed convex surfaces defining adjacent venturi sections with a common axis and an annular depression therebetween. Steering and maneuvering of barges with such a unit has proven quite successful except that it has been found difficult to prime the pump blades with water during initial movement of the barge or when under way particularly at high speeds during both a loaded or deep draft condition and during an unloaded or light draft condition. It should be noted that the waterline of the watercraft is normally above the top of the pump blades during a loaded or deep draft condition, and that the waterline of the watercraft is normally below the top of the pump blades during an unloaded or light draft condition.
As the craft moves through the water, a cushion of air moves along the bow rake until it reaches the nozzle openings as in my above-noted U.S. Pat. No. 3,590,766 whereupon it enters such openings thus displacing water in those areas as well as in the vicinity of the pump unit. As travel of the vessel continues, air build-up continues until enough water is displaced to cause a loss of priming in the vicinity of the pump blades. And, as the travel speed of the vessel increases, the increased velocity of the water moving across the nozzle openings causes an additional problem of negative pressure build-up across the nozzle openings thereby further complicating the free passage of water into the pump unit. The effectiveness of the steering unit is accordingly decreased.
The above conditions are apparent for a deep draft condition of the vessel and are even more amplified for a light draft condition with the waterline below the top of the pump blades creating an additional under-priming situation and the acceleration in vessel travel thereby compounding the problem of the negative pressure preventing free passage of water into the pump unit.
The under-priming problems as noted above are not limited to barges or the like wherein the inlet and outlet nozzles intersect with spaced openings provided in the bottom hull. Other types of watercraft have been designed with a transversely disposed water channel extending between openings provided in opposing side walls of the craft with the channel located entirely below the waterline of the craft. An axial flow pump disposed in the channel is designed to move water from one end thereof to the other for steering the vessel. Such an arrangement is disclosed in U.S. Pat. No. 3,874,316. However, under-priming problems similar to those noted above are presented for such an arrangement. The problem of overcoming the vacuum forces created by rapid movement of the water moving past the nozzle openings is likewise of concern for such a particular steering arrangement.